TweetFollow Us on Twitter

January 92 - The Soup Kitchen - Extending C++ with C++

The Soup Kitchen - Extending C++ with C++

Eric M. Berdahl

This month I'll look at C++ from a slightly different perspective. One of my central themes has been object code compatibility with Pascal and MacApp. But this will not be an issue today. Instead, I'll be looking at tools intended to make the C++ programmer's life easier by extending the language with "pure" C++ classes.

THE MEANING OF "EXTENDABLE"

Most languages are judged, at least initially, by the support they provide for certain "key" issues. String manipulation is a typical example of such an issue.

Support for strings in C and Pascal marks two extremes of a spectrum. Pascal provides rather intuitive syntax for dealing with strings, while C provides for support via a powerful but non-intuitive library of functions. The advantage of standard C is the programmer's ability to easily create superior string handling functions at the cost of readability. The advantage of standard Pascal is the programmer's ability to create very readable code at the expense of flexibility.

C++ provides something between these C and Pascal. C++ doesn't have built-in support for strings, but the class structure and messaging protocol allows you to design string classes that are intuitive and easy to change. When this is done, you can use strings as if C++ has predefined definitions for what strings are and how to manipulate them. Furthermore, C++ lets you create small classes whose sole purpose is to aid in housekeeping chores or other code-level cleanup that is sometimes required when programming complex systems like the Macintosh.

HOUSEKEEPING À LA C++

General techniques One of the more tedious tasks facing Macintosh programmers is the necessity to save different states at various points in one's code. MacApp does nothing but add to the complexity of the problem. Lock state of handles, pen states, focused views, and permanent allocation flags are simple examples of things MacApp programmers need to keep track of and keep in sync with code.

The classic approach to this problem is to write code resembling the following:

{
    StateRecord aState;
    GetCurrentState(aState);
    //  Insert code here that screws around
    //  with the element we want to save
    SetCurrentState(aState);
}

This code is straightforward and works fine. However, it will run into problems when you need to contend with MacApp failure handling. In this situation, you need to provide failure handling code to restore the state appropriately, as follows:

{
    StateRecord         aState;
    FailInfo            fi;
    GetCurrentState(aState);
    if (fi.Try())
    {
        //  Insert code here that screws around
        //  with the element we want to save
        SetCurrentState(aState);
        fi.Success();
    }
    else
    {
        SetCurrentState(aState);
        fi.ReSignal();
    }
}

This code is functional and perfectly acceptable, though rather wordy. It uses a simple concept-it preserves the state across this section of code. This bit of housekeeping is a perfect candidate for an object.

The technique I describe here applies to all forms of housekeeping. But for sake of brevity, I'll restrict my explication to one application-locking a handle.

CHandleLocker
class CHandleLocker
{
private:
    Handle      fHandle;
    Boolean     fWasLocked;
    FailInfo    fFailInfo;
protected:
    pascal void HandleFailure(OSErr err, long message);
    virtual pascal RestoreLockState();
public:
    CHandleLocker(Handle theHandle);
    virtual ~CHandleLocker();
};

CHandleLocker is a simple class that assures a handle remains locked within a particular scope and that it reverts to its previous state outside that scope. Using CHandleLocker, you can write code that utilizes handle locking in the following manner:

{
    CHandleLocker   lockSomeHandle(someHandle);
    //  Insert code that relies on someHandle being locked
}

Magically, when this code block executes, someHandle will be locked. The intervening code will execute, and someHandle will be returned to the state it was in (either locked or unlocked) when the block began. As an added bonus, if a MacApp failure is raised somewhere in the intervening code, someHandle will still be left in its original state. Amazingly, CHandleLocker adds safe handle locking to C++, and gives it a syntax that is relatively simple and unobtrusive.

Under the hood of CHandleLocker

To accomplish this little bit of heaven, CHandleLocker relies on the constructor and destructor mechanisms of C++ classes. The constructor and destructor are the core of the class. The constructor is called automatically when an instance is created, as in the declaration of lockSomeHandle in the example above. The destructor is called when an instance is destroyed, as in the closing curly brace in the same example.
CHandleLocker::CHandleLocker(Handle theHandle)
{
    fHandle = theHandle;
    fWasLocked = IsHandleLocked(fHandle);
    if (!fWasLocked)
        HLock(fHandle);
    CatchFailures(fFailInfo, CHandleLocker::HandleFailure, this);
}

CHandleLocker::~CHandleLocker()
{
    Success(fFailInfo);
    this->RestoreLockState();
}

The constructor simply saves the handle and its original lock state in instance variables. If the object is originally unlocked, it locks it. Finally, a MacApp failure handler is installed. In the destructor, the failure handler is removed and the lock state is restored. The remaining parts of the class are the method to restore the lock state and the failure handler:

void CHandleLocker::RestoreLockState()
{
    if (!fWasLocked)
        HUnlock(fWasLocked);
}

pascal void CHandleLocker::HandleFailure(OSErr err, long message)
{
    this->RestoreLockState();
}

RestoreLockState is straightforward. It simply refers to the fWasLocked boolean instance variable and unlocks the handle if it was originally unlocked. But the failure handler is a bit more involved and will take a short explanation.

CHandleLocker::HandleFailure is mediated using the Pascal-style failure handling routines, CatchFailures and Success. Therefore, the failure handling routine does not need to resignal the failure since returning from the routine automatically invokes the next failure handler on the stack.

Also, the method is declared with the keyword pascal, indicating that we want the return values and arguments passed Pascal-style. This is important once you recognize that methods with Pascal-style arguments will have the metavariable this passed as the last "argument" of the routine, the same position the Pascal static link variables occupy (see the September installment of The Soup Kitchen for a discussion of nested procedures and static links). Notice that this is passed as the static link variable in the CatchFailures argument list above.

That's all there is to this simple little utility class. I challenge the reader to write utility objects to handle other housekeeping chores. To get started, look at the PenState of QuickDraw, and the focus information in the MacApp view architecture. It should be fairly simple to use CHandleLocker as a template to build a useful library of magic little objects.

STRINGS

Admittedly, utility classes are simple examples and do not come close to making the case for language extensibility. However, having opened the door and provided some decent groundwork, let's take a look at CStr255, a very nice piece of work from the MacApp Team at Apple.

Any Macintosh programmer is familiar with the basic Pascal string. The Pascal string differs from the C string in the following ways:

  • Pascal strings are limited to 255 characters; C strings are basically unlimited in length.
  • Pascal strings contain the length of the string in their first byte; C strings are terminated with a null byte (hex 00).
  • Pascal strings are supported with Pascal language syntax; C strings are supported by convention and a standard C library.

Pascal programmers are fortunate to be able to use standard Macintosh strings, Str255, with little or no effort. Since these strings are nothing more than real Pascal strings, the compiler and run time environment take care of the details. The C++ programmer has the ability to use Pascal-like syntax to define and manipulate Pascal strings, but must be constantly aware of the difference and must maintain discipline to convert to the appropriate format as necessary.

Unlike the example of maintaining the lock state of a handle, writing a fully capable language-extending class like CStr255 is beyond the skill of all but master C++ architects. CStr255 provides an excellent example of what C++ can do, but it should not be imitated by the novice. This warning is sorely lacking in all C++ books I've encountered and is the primary cause of problems with novice and intermediate level C++ programmers. To quote Barnum and Bailey, "Kids, don't try this at home."

In its final form, CStr255 allows you to write code that looks like this:

{
    CStr255 aString;
    aString = "A string";
    aString = aString + "another string";
    if (aString == anotherString)
        gApplication->Beep(3);
    theLength = aString.Length();
    theFifthCharacter = aString[5];
    DrawString(aString);
    DrawString("A string constant");
    printf("This line prints C strings %s", (char*)aString);
}

In short, you can write code that looks like Pascal but that integrates well with your C++ code.

Constructor magic

The first rabbit I'll pull out of CStr255 involves its constructors. The designers of CStr255 provide a constructor that takes a single char* as an argument: CStr255::CStr255(char*).

Since all standard C strings and string constants in C++ are of type char*, this means you can assign C strings and string constants directly to any variable of type CStr255. Furthermore, any routine that takes a CStr255 as an argument, like DrawString, can be passed a C string or string constant with no difficulty.

Similarly, CStr255 provides for constructors from other Macintosh string types (CStr63 for example), other C types (like unsigned char*), and even a constructor from a longint that is used to convert ResType and OSType variables to their string equivalents. As with all the CStr255 code I'll quote from here, these constructors are found in the file PascalString.h in the MacApp 3.0b3 release.

Operator magic

Constructors are handy and useful, but without the operators +, =, ==, and others, CStr255 would just be a neat utility class. Instead, with these operators, you have access to a truly powerful class that adds Pascal strings to the C++ language with a very simple and intuitive syntax. Since it's not reasonable to reproduce the CStr255 code here, I'll leave you with a taste of the features available for string handling and short examples of their utility.

Constructors

CStr255();                            //  CStr255 aStr
CStr255(char*);                       //  aStr = "A string"
CStr255(const CStr255&);          // aStr = anotherStr255
CStr255(const CStr63&);           //  aStr = aStr63
CStr255(long);                        //  aStr = 'PICT'

Operators

char& operator[](short pos);      //  aStr[4] = 'P'
CStr255 operator+(CStr255&);      //  aStr = bStr + cStr
CStr255 operator=(CStr255&);      //  aStr = bStr
Boolean operator==(CStr255&);     //  if (aStr = bStr)
Boolean operator==(const char*);      //  if (aStr == "A string")
Boolean operator>(const char*);    //  if (aStr > "ABC")

Handy functions

char& Length();                   //  aStr.Length() = 7
Boolean IsEmpty();                    //  if (aStr.IsEmpty())

Conversions

operator char*();                     //  printf("%s", aStr);
operator long();                      //  GetResource(long(aStr), kID);
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

xScope 4.1.2 - Onscreen graphic measurem...
xScope is powerful set of tools that are ideal for measuring, inspecting, and testing on-screen graphics and layouts. Its tools float above your desktop windows and can be accessed via a toolbar,... Read more
MacFamilyTree 7.3.3 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more
Skype 7.5.0.738 - Voice-over-internet ph...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
PushPal 3.0 - Mirror Android notificatio...
PushPal is a client for Pushbullet, which automatically shows you all of your phone's notifications right on your computer. This means you can see who's calling or read text messages even if your... Read more
Logic Pro X 10.1.1 - Music creation and...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more
VLC Media Player 2.2.0 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Sound Studio 4.7.8 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
LibreOffice 4.4.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VueScan 9.5.03 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Freeway Pro 7.0.3 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more

This Week at 148Apps: February 23-27, 20...
Final February Fun at 148Apps   How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you’re looking for. The ones we love become Editor’s Choice, standing out... | Read more »
GDC 2015 – Does Not Commute is Definitel...
GDC 2015 – Does Not Commute is Definitely a Game You Should Keep an Eye on Posted by Rob Rich on March 2nd, 2015 [ permalink ] We were teased about Mediocre Games’ (Smash Hit, | Read more »
F84 Games & POW! Announce Stan Lee V...
F84 Games has announced that it is working with legendary comic creator Stan Lee and POW! Entertainment to produce Stan Lee’s Hero Command. The game will be a action adventure of heroic proportions. | Read more »
Setlyst Keeps Your Set Straight So You C...
Setlyst Keeps Your Set Straight So You Can Focus On Rocking Out. Posted by Jessica Fisher on March 2nd, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Space is Vast, So Space Agency Has a Vas...
Space is Vast, So Space Agency Has a Vast New Update! Posted by Jessica Fisher on March 2nd, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Size DOES Matter Review
Size DOES Matter Review By Campbell Bird on March 2nd, 2015 Our Rating: :: HARD TO BEATUniversal App - Designed for iPhone and iPad This rhythm game has a unique control scheme and performance system that make it feel like a true... | Read more »
The first ever action 3D card battler Al...
On the other hand, you probably haven’t played an action 3D card battler – until now. Step forward, All Star Legion. All Star Legion is a 3D QTE-based action RPG card battler, but fear not – the game itself isn’t as convoluted as its description.... | Read more »
Travel Back to the 1980s With the Making...
Headup Games has released a hilarious making of video for its upcoming title, Pixel Heroes: Byte & Magic. The game is a RPG/Roguelike where you control three heroes set to save the township of Pixton from an evil cult called The Sons of Dawn.... | Read more »
Heavenstrike Rivals Review
Heavenstrike Rivals Review By Campbell Bird on March 2nd, 2015 Our Rating: :: HEAVENLY STRATEGICUniversal App - Designed for iPhone and iPad Despite a few flaws, this free-to-play strategy game is a fun mix of new and old strategy... | Read more »
Get The Whole Story – Lone Wolf Complete...
Get The Whole Story – Lone Wolf Complete is Now Available and On Sale Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

Sale! 15-inch 2.2GHz Retina MacBook Pro for $...
 Best Buy has the 15″ 2.2GHz Retina MacBook Pro on sale for $1774.99 $1799.99, or $225 off MSRP. Choose free home shipping or free local store pickup (if available). Price valid for online orders... Read more
27-inch 3.5GHz 5K iMac in stock today and on...
 B&H Photo has the 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available for... Read more
Apple Launches Free Web-Based Pages and Other...
Apple’s new Web-only access to iWork productivity apps is a free level of iCloud service available to anyone, including people who don’t own or use Apple devices. The service includes access to Apple... Read more
Survey Reveals Solid State Disk (SSD) Technol...
In a recent SSD technology use survey, Kroll Ontrack, a firm specializing in data recovery, found that while nearly 90 percent of respondents leverage the performance and reliability benefits of SSD... Read more
Save up to $600 with Apple refurbished Mac Pr...
The Apple Store is offering Apple Certified Refurbished Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Apple CEO Tim Cook to Deliver 2015 George Was...
Apple CEO Tim Cook will deliver the George Washington University’s Commencement address to GWU grads on May 17, at which time he will also be awarded an honorary doctorate of public service from the... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Save up to $50 on iPad Air 2s, NY tax only, f...
 B&H Photo has iPad Air 2s on sale for $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469.99 $30 off - 64GB iPad Air 2 WiFi: $549 $50 off - 128GB iPad Air 2... Read more
16GB iPad Air 2 on sale for $447, save $52
Walmart has the 16GB iPad Air 2 WiFi on sale for $446.99 on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only,... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Pay Automation Engineer - iOS System...
**Job Summary** At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring passion and dedication to your job Read more
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand Read more
*Apple* Pay - Site Reliability Engineer - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.